Fluorescent lamps require mercury to operate. Because of mercury's perceived environmental problems, recent regulatory controls impose lower and lower mercury dosing in fluorescent lamps. As these doses decrease, they approach the minimum dose required to operate the lamp over its projected lifetime. It has proven to be very difficult to accurately maintain the very small doses necessary to meet environment constraints while ensuring consistent lamp quality and life.
Fluorescent lamps have been (and still are) dosed with a variety of techniques. Liquid dosing is the simplest and least expensive method; however, it is very inaccurate and virtually impossible at doses lower than 4.5 mg, especially when lamps are processed on high-speed equipment.
In attempts to solve the dosing or dispensing of mercury, industry has used a variety of glass and metal capsules. These techniques offer several advantages, for example, the accuracy and size of the dose is only limited by the mercury metering and delivery equipment used to place the mercury in the capsule. Since these techniques can be run off-line at a separate facility, slow and accurate filling methods can be employed. However, the disadvantages include the fact that the capsules must be mounted on a structure within the lamp, thus adding to the cost and complexity. Further, the capsule must be opened within the lamp after the lamp has been evacuated and the exhaust tube sealed, adding a processing step and the potential for additional lamp failures.
Additional procedures have used the placement within the lamp of a strip of material containing a titanium/mercury alloy that decomposes at temperatures near 900 degrees C. However, the variation in mercury dose from strip to strip is large enough that dosing at amounts less than 2.5 mg is not practical. Also, like the capsules, the strip must be mounted within the lamp in a predictable manner and be activated by an external radio frequency field.
Recently, it has been proposed (U.S. Pat. Nos. 6,905,385 and 6,913,504) that dosing could be accomplished by coating a steel ball with silver and subsequently applying mercury to the silver coating. While this technique provides relatively accurate control over the amount of mercury, it has been found that if the steel ball remains loose in the lamp, it causes damage to the phosphor coating. Further, after manufacture, it is necessary to keep the mercury/silver coated balls separated since it has been found, through testing, that allowing the balls to come into contact with one another allows for the transfer of mercury between them, thus destroying the necessary accuracy for dosing requirements.